Mads Jakob Herring Jensen

Bio: Mads Jakob Herring Jensen is an academic researcher from Technical University of Denmark. The author has contributed to research in topics: Bubble & Microchannel. The author has an hindex of 8, co-authored 18 publications receiving 553 citations. Previous affiliations of Mads Jakob Herring Jensen include Columbia University.

A numerical study of microparticle acoustophoresis driven by acoustic radiation forces and streaming-induced drag forces

Abstract: We present a numerical study of the transient acoustophoretic motion of microparticles suspended in a liquid-filled microchannel and driven by the acoustic forces arising from an imposed standing ultrasound wave: the acoustic radiation force from the scattering of sound waves on the particles and the Stokes drag force from the induced acoustic streaming flow. These forces are calculated numerically in two steps. First, the thermoacoustic equations are solved to first order in the imposed ultrasound field taking into account the micrometer-thin but crucial thermoviscous boundary layer near the rigid walls. Second, the products of the resulting first-order fields are used as source terms in the time-averaged second-order equations, from which the net acoustic forces acting on the particles are determined. The resulting acoustophoretic particle velocities are quantified for experimentally relevant parameters using a numerical particle-tracking scheme. The model shows the transition in the acoustophoretic particle motion from being dominated by streaming-induced drag to being dominated by radiation forces as a function of particle size, channel geometry, and material properties.

A numerical study of two-phase Stokes flow in an axisymmetric flow-focusing device

Abstract: We present a numerical investigation of the time-dependent dynamics of the creation of gas bubbles in an axisymmetric flow-focusing device. The liquid motion is treated as a Stokes flow, and using a generic framework we implement a second-order time-integration scheme and a free-surface model in MATLAB, which interfaces with the finite-element software FEMLAB. We derive scaling laws for the volume of a created bubble and for the gas flow rate, and confirm them numerically. Our results are consistent with existing experimental results by Garstecki et al. [Phys. Rev. Lett. 94, 164501 (2005)], and predict a scaling yet to be observed: the bubble volume scales with the outlet channel radius to the power of 4 and the surface tension. Our axisymmetric simulations further show that the collapse of the gas thread before bubble snap-off is different from the recent experimental results. We suggest that this difference is caused by differences in geometry between experiments and the simulations.

The clogging pressure of bubbles in hydrophilic microchannel contractions

Abstract: We present a theoretical and numerical study of the quasi-static motion of large wetting bubbles in microfluidic channels with contractions In most cases the energy of a bubble increases when it is moved from a wide channel to a narrow one, and the bubble thus tends to clog the flow of the fluid A certain pressure, the so-called clogging pressure, is needed to push the bubbles out of the contraction However, we show that in the case of a hydrophilic channel contraction there exists a range of parameter values where the bubble actually gains energy by moving into the narrow part For these specific cases we analyze how the clogging pressure depends on channel geometry, surface tension and contact angle Based on our analysis we establish design rules for minimizing the clogging pressure of microchannel contractions

A novel electro-osmotic pump design for nonconducting liquids: theoretical analysis of flow rate-pressure characteristics and stability

Abstract: We present the design and theoretical analysis of a novel electro-osmotic (EO) pump for pumping nonconducting liquids. Such liquids cannot be pumped by conventional EO pumps. The novel type of pump, which we term the two-liquid viscous EO pump, is designed to use a thin layer of conducting pumping liquid driven by electro-osmosis to drag a nonconducting working liquid by viscous forces. Based on computational fluid dynamics, our analysis predicts a characteristic flow rate of the order nL/s/V and a pressure capability of the pump in the hPa/V range depending on, of course, achievable geometries and surface chemistry. The stability of the pump is analyzed in terms of the three instability mechanisms that result from shear-flow effects, electrohydrodynamic interactions and capillary effects. Our linear stability analysis shows that the interface is stabilized by the applied electric field and by the small dimensions of the micropump.

Transient pressure drops of gas bubbles passing through liquid-filled microchannel contractions: an experimental study

Abstract: We present the first transient pressure measurements and high-speed visualization of gas bubbles passing through liquid-filled microchannel contractions. We have studied contractions ranging from 100 to 500 ?m in glass tubes of main diameter 2 mm and compared the experimental results with the recent model of quasi-stationary bubble motion by Jensen, Goranovi? and Bruus (2004 J. Micromech. Microeng. 14 876) valid for low flow rates. The influence of the wetting angle is studied by coating a tube with a hydrophobic solution. Transient pressure measurements, bubble deformations and the influence of the bubble length on the so-called clogging pressure ?Pc are shown to be in good agreement with the model, both in terms of maximum values and in terms of transient evolution. Some deviations from the model are also observed and possible reasons for these are investigated, such as (a) contact line pinning, (b) thin liquid film along the bubble modifying capillary pressure and (c) viscous pressure drop in the contraction. Experiments with increasing flow rates show that two regimes govern the pressure transients of the bubbles passing the contractions: a quasi-stationary regime for low capillary number and a viscosity-influenced regime for non-negligible capillary numbers. We propose a criterion based on a modified capillary number to discriminate between these two regimes.

Reactions in Droplets in Microfluidic Channels

Abstract: Fundamental and applied research in chemistry and biology benefits from opportunities provided by droplet-based microfluidic systems. These systems enable the miniaturization of reactions by compartmentalizing reactions in droplets of femoliter to microliter volumes. Compartmentalization in droplets provides rapid mixing of reagents, control of the timing of reactions on timescales from milliseconds to months, control of interfacial properties, and the ability to synthesize and transport solid reagents and products. Droplet-based microfluidics can help to enhance and accelerate chemical and biochemical screening, protein crystallization, enzymatic kinetics, and assays. Moreover, the control provided by droplets in microfluidic devices can lead to new scientific methods and insights.

Microfluidic methods for generating continuous droplet streams

Abstract: Microfluidic technologies have emerged recently as a promising new route for the fabrication of uniform emulsions. In this paper, we review microfluidic methods for synthesizing uniform streams of droplets and bubbles, focusing on those that utilize pressure-driven flows. Three categories of microfluidic geometries are discussed, including co-flowing streams, cross-flowing streams, and flow focusing devices. In each category we summarize observations that have been reported to date in experiments and numerical simulations. We describe these results in the context of physical mechanisms for droplet breakup, and simple theoretical models that have been proposed. Applications of droplets in microfluidic devices are briefly reviewed.

Transition from squeezing to dripping in a microfluidic T-shaped junction

Abstract: We describe the results of a numerical investigation of the dynamics of breakup of streams of immiscible fluids in the confined geometry of a microfluidic T-junction. We identify three distinct regimes of formation of droplets: squeezing, dripping and jetting, providing a unifying picture of emulsification processes typical for microfluidic systems. The squeezing mechanism of breakup is particular to microfluidic systems, since the physical confinement of the fluids has pronounced effects on the interfacial dynamics. In this regime, the breakup process is driven chiefly by the buildup of pressure upstream of an emerging droplet and both the dynamics of breakup and the scaling of the sizes of droplets are influenced only very weakly by the value of the capillary number. The dripping regime, while apparently homologous to the unbounded case, is also significantly influenced by the constrained geometry; these effects modify the scaling law for the size of the droplets derived from the balance of interfacial and viscous stresses. Finally, the jetting regime sets in only at very high flow rates, or with low interfacial tension, i.e. higher values of the capillary number, similar to the unbounded case.

Recent advances in microscale pumping technologies: a review and evaluation

Abstract: Micropumping has emerged as a critical research area for many electronics and biological applications. A significant driving force underlying this research has been the integration of pumping mechanisms in micro total analysis systems and other multi-functional analysis techniques. Uses in electronics packaging and micromixing and microdosing systems have also capitalized on novel pumping concepts. The present work builds upon a number of existing reviews of micropumping strategies by focusing on the large body of micropump advances reported in the very recent literature. Critical selection criteria are included for pumps and valves to aid in determining the pumping mechanism that is most appropriate for a given application. Important limitations or incompatibilities are also addressed. Quantitative comparisons are provided in graphical and tabular forms.